System for analyzing and synthesizing speech



March 16, 1954 R. c. MATHES SYSTEM FOR ANALYZING AND SYNTHESIZING SPEECH Filed Feb. 2, 1949 5 Sheets-Sheet l TORN R. C. MATH ES March 16, 1954 SYSTEM FOR ANALYZING AND SYNTHESIZING SPEECH 5 She't's-Sheet 2 Filed Feb. 2, 1949 VENTO@ BV R. @Y MA rHEs QM 7' TORN V R. c. MATHES 2,672,512

SYSTEM FOR ANALYZING AND SYNTHESIZING SPEECH 45 Sheets-Sheet 4 March 16, 1954 Filed Feb. 2, 1949 Marchs, A1954 Filed Feb. 2, 1949 VOL TA GE R. c. MATHEs 2,672,512

sYsTEM EOE ANALYZING AND sYNTEEsIzING SPEECH 1 5 sneetssheet 5 4 f f l i L ci L TIME ll Il Fl lil Il F/G-g3 /A/ VEN TOR R. C. MA THES Patented Mar, 16, 1954 SYSTEM FOR ANALYZIN G AND SYNTHESIZING SPEECH Robert C. Mathes, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application February 2, 1949, Serial No. 7 4,064

2l Claims. l

The present invention relates to signal wave transmission systems, and more particularly to such systems wherein a complex wave is analyzed to determine its fundamental characteristics.

The invention is particularly adapted to providing an improved type of harmonic analysis of a complex wave whose fundamental frequency is varying. It is concerned with securing accurate information concerning the amplitudefrequency characteristic of signal waves, which information may then be used in lieu of the original signal frequencies for the transmission of the message intelligence.

Although the invention is `particularly applicable to the transmission of speech signal waves, its utility is not limited to such signal waves, since it may be used with equal facility to transmit the intelligence of any signal wave of a periodic or quasi-periodic type.

In a specific embodiment of the invention, which will be described hereinafter, the invention is shown as being incorporated in a vocoder transmission system such, for example, as was disclosed in H. W. Dudley Patent 2,151,091, March 2l, 1939. The present invention is in the nature of an improvement on certain aspects of such a system.

In the vocoder message transmission system, for example, the system that is disclosed in the previously mentioned Dudley patent, the message Waves are divided into two portions and these portions are then analyzed to determine certain of their fundamental characteristics. One of these portions is subjected to a process by which the original fundamental frequency of the signal Wave is determined. The product of this frequency determining process may be a unidirectional voltage the magnitude of which slowly varies at a rate corresponding to the syllabic rate of the message signal. may be transmitted directly to the receiving station for use in the signal reproduction process, or it may be used to control the magnitude of a single frequency alternating voltage which may, in turn, be transmitted to the receiving station for use in the reproduction process, as is explained in the Dudley patent.

The second portion of the divided signal wave is further divided into a iixed number of frequency sub-bands. quency bands is then subjected to a detection process in which the signal energy falling within that band is integrated. This process produces a number of slowly varying currents, or voltages, the magnitude of each being representative of This voltage t Each of these narrow frethe signal energy in a corresponding frequency sub-band at any given instant. These slowly varying currents, which collectively form the socalled amplitude pattern control current, are also transmitted, either in their derived state, or as modulated distinctive frequency voltages, to the receiving station Where they control the amplitude-frequency distribution of the reproduced signal wave.

At the receiving point, these signal-defining currents, or voltages, are utilized to (a) control the fundamental frequency of a source of socalled voiced energy, and (b) control the amplitude distribution of the synthesized signal energy within the frequency band of the reproduced signal. For a more complete understanding of the operation of this vocoder system, reference may be had to the previously mentioned Dudley patent.

In signal transmission systems of the type which has just been generally described, the original signal energy is subdivided into narrow frequency bands through the use of a number of fixed frequency band filters. Because the passband of each of these iilters is a iixed quantity, and because the location of the prominent harmonic components of the signal waves is not a fixed quantity, it frequently happens that an important harmonic component of the signal frequency will occur at or near a frequency which corresponds to the cross-over point on the attenuation curves of two adjacent band filters. Since these cross-over points occur at the edges of the pass-bands of the filters at points where the attenuation characteristics of the filters are changing relatively rapidly, the coinciding harmonic frequencies are unduly attenuated, and are also subjected to phase variations which are not proportional to frequency. This means that, as the pitch of the signal varies, one or more of the harmonic components of the wave may have undesirable amplitude and phase modulations superposed upon it, which modulations are reproduced in the synthesized signal. rihis effect may occur at each cross-over point within the signal frequency band, and hence has been called the comb eifect.

A second undesirable effect that may arise because of the xed character of the pass-bands of the segregating band lters occurs in the synthesizing operation at the receiving end of the system. At the receiving end a source which is rich in harmonic components, and in which adjacent components are of substantially the same magnitude, supplies the energy for the syntheanequalhumber ofA "quency components -as those' existing sized voice signal. The receiving end band lters transmit all harmonic frequencies which fall Within their pass-band with about equal facility. The amplitudes of these passed harmonies are then modulated in accordance with their respective signal-defining control currents in such manner that .all harmonics within=an individual pass-band have substantially' the same amplitude, which amplitude is the integrated value of all of the harmonics in the corresponding band of the signal. This conditionjproduces a synthesized signal which is made uprofeafnumber of frequency components in substantially harmonic distribution, and in Vvsrhich the vamplitudes of the harmonic frequencies-within a group are substantially equal, instead of a signal in which each harmonic componentfhas 7theisame relative amplitude as it had in the original signal. This relationship has been characterized as the bloc eiect, and is believed to impart a raucous qua'lity to thesynthesized'signaL 'Itfisaccordin'glr n object ofthis` inventionto improvethe implitudefrequencyanalysis ofcomplex waves,WhichWaves-'may include a'number 'of 'harmonically related frequency components. It' is also an object of this invention -to= deter- 'mine the ampltudeefrequency lcharacteristic l' of the :harmonic components "of a -complex `Wave Without introducing undesirable amplitude or 'iphase'chan'ges `du'ringthefanalysis -JIt'is also`anfbj'ect of this invention to analyze 'a complextwaveiin accordance With-the number *of harmonicrcom'ponents that arepresent in the 'Wave "at: any given iinstant. 4AAsthe number of `.harmonic components varies from one-instant to anothenit is'an object of this invention to'vary accordingly" thenumber of amplitude-frequency relationships that' are determined.

It is also an object of this invention'-to-im provethe'quality of thesynthesized-signal of a "voco'd'er ltransmission system Vby'freconstruct-.ing L'the' signal from"harmonically "related frequency components which areat all times nequal finnumber to and L in "substantially the `salme order r `of rJamplitudes'as corresponding'-harmonically related "frequency components the original signal wave.

It a i feature "'of the ""present invention that segregating 'means' "aref utilized the' analysisfof 'ja `mom'pl'ex A`signal Wave' whereby"v the individual ""harmonic' components 'of' thewave are separately segregatedby'an 'individual means notwithstandingthat each-component 4var-iesl its p-ositionin `rthe frequencyspectrum from instant; to instant.

variable number of'variablef frequency segregating means f are -utilized at the synthesizing lpoint "to selectirom` thesourceof' synthesizing energy harmonically f Irelated frein the -original wave.

A 'It `Ais Ya'A further` feature of the-invention that "falth'ough the number f of 'harmoncally related -frequ'encycomponentsl thatare VAanalyzed-is a "variable quantity,Av and although 'the 4number f. of i 1'harrnonically"related: frequency' components"y that flare' selectedfatlthefsynthesizlng.station ris variable from instant to instant, thenumberzbtzandfhand v'vidthof the transmissionh 'channels interconnectl ingi theaanalyzing "anda synthesizing stations: -re- 'zmainfixed.

The :invention .eis further: featured bym axspecimenfatakingsor samplingcprocesadn whichtche W variable number. oi 'segregated isignalvcom'pcnents ffarefsampledffatf; a; r ,cyclical frecu'rrence,z asatecthat is ffprUpor-tional #to thermaximummurnberiof :ampli-A tude changes in the signal, While at the same time the interval between samples of adjacent components is varied in accordance with changes in the pitch of the original signal.

Other desirable objects and features of the invention, and the manner in which they are .f realized, .yvillbe, apparent-iromL the following de 1vtailed "description of one fembodimentlof the invention, when considered in connection with the '110 drawings, in which:

-Eig.1.shovvs a block schematic diagram of the fana'lyzingfend of a vocoder transmission system, the amplitude-frequency analyzing branch of Whichhas'-beenl-arranged in accordance with the 51"'5 invention;

PFig.' 2 isa"bloc1114 schematic diagram of the re- -.ceiving,orasynthesizing end of a vocoder transmission system in which the receiving amplitude- 'frequency control branch of the system is arranged in accordanceawth the invention;

'l Figs: 3, 4,25, Afand :8B,;ar,e.f.i1lustrative graphs z-torwhiclrreierenceis made. imthedetailed descrip- .'tionf'oithe invention;

aFig. .6 t` is ya ":diagramaof the variable i frequency 25 vfsegregating :means employedzat` thertransmitting end of; thefsystem comprisedindiigsll yar1d2,;

AiFig. -7 is ffafschematic adiagram :0f fthe variable @speedfA enablingl circuit :ofgthe invention;

,@Fifg. ,-.191isf1a schematiczdagramrof sa variable .eo speed .samphnemmgate:circuitwhich mavbeutilized .'infzconnection with'rthe invention; and

zfFigsrlOA, :.lOBtland ,11 rare; schematic diagrams of band-pass nIter :structures may vbe adaptedLas;thev variable: seleetivefimeansfas; alter- ,fnative 2 arrangements; for the a variable frequency :fselectiveirmeansf,illustrated in Flei.

i accordance with this rinvention, the mes- .zsage signal wvlavelisfsubdivided into .'two, portions for fundamentalepitchaandffamplitudeffrequenv A u @determining purposes. j-fEhe ;:pitch,defming -por tionlis rutilizedftogderivegaunidirectional voltage sthe;aimantadencftwhiahvaries :at ,adslowfeyllabic f'rate;.and;which isfi-ndieativeff the instantaneous fnndamental requenoy i .of :the message signal. 5 Thisiumdirectionahvoltage.servesthreepurposes- -One is;the:contro1f:cf the'iunda-mental frequency of the receiving end source of synthetic voiced cenergy. :Anothernis thercontrolxof the. frequency fsettinguot.;acpluralitygofvariablefrequencyssegrezegatingsmeans. 'Bheuthirdfisthe; fcontrolf of the ispeedcor; frequency L of frecurrence, iv/.ith .Wh-ich samples are ptakencof thef emerga-.selected from z.the.signal-wavenby i eachuof these variable .IfrencluencyrA segregatingnmeans;.'.atgthe;3analyzing. end aand". rhe-,samplingcoffaxspectrumrampltudevoltcageflatztherreceivingrend of the system. l'Ihe speciicfV manner.inwhich' these control; purposes gare: eiectedfwillnbe ilaterudescribed; finr considervfablev detail.

@Theasecondfportionio;thewriginaksignalwave ris; analyzed ,fintthef;amplitudeffrequenyf, Control xbranchrg;ofxiihessystem. .Thisgbranchincludes a vvariableinequencywsegregating; circuit for selectling each cindividual :.harmonically fr eleted :ire- 5.quencyrccmnponent thatzzmayaber,present-,inthe :rsignalcwave yEachssegregatingi:means: included in this circuit hasaa,':variablefrfnequencycharaci teristicawhich 1 is :controlled: by-.Lthe pitch-defining lelunidirectional voltage referrednzto:previously i'nhesafselected energies, correspcndinetto-fthe several narmonicmomponents .fin the signal-wave, ina-re4 dirented;.in.to individualcci-rcut .schamele vai/:here theyiare stoned. Specimensof Vthesewener- .5. gies: are recurrentlyltakenfbyasampling, v orgl gatingr.arrangement-whichfislalso unden-theeolltrol of the previously mentioned pitch-defining voltage. This sampling circuit is so controlled that it performs a complete sampling cycle during a xed interval that is proportional to the maximum number of signal changes in a second, but it varies its sampling rate during any cycle in a manner that is inversely proportional to the pitch of the signal at the corresponding time. Therefore, during any interval of time, the number of samples taken is inversely proportional to the fundamental frequency of the signal wave during that interval. Stated otherwise, the number of samples is equal to the number of harmonic components that are present in the signal frequency band. These individual samples are combined into a composite voltage function that portrays an amplitude-frequency time representation of the envelope of the original signal energy. This composite representation may be termed a spectrum-amplitude voltage which is representative of the relative amplitudes of the harmonic frequency components that are present in the signal Wave during any interval.

At the synthesizing station or terminal of the system this spectrum amplitude-defining voltage is sampled by a variable speed sampling, or gating arrangement, the samp-ling frequency of which is under the control of the pitch-defining voltage in the same manner as is the speed of the sampler at the analyzing station or terminal of the system. This insures that the spectrum-amplitude curve, or voltage, shall be sampled the same number of times and in the same relative time order as the energy samples occurred at the analyzing point in constructing this spectrum voltage. Variable frequency segregating means, corresponding in number and frequency characteristics to the variable frequency segregating means at the analyzing terminal are controlled in their operation by the pitch-defining voltage such that they select from the source of synthetic voiced signal energy, frequency components which have the same relative frequency spectrum positions as did the harmonically related frequency components in the original signal wave. The output of each Variable segregating means is supplied to a shaping network along with an amplitude control voltage that is obtained from the spectrum-amplitude voltage representation. In this manner, the reproduced, or synthesized, signal is composed of an equal number of harmonically related frequency components of the same fundamental frequency as existed in the original signal Wave at any given instant. Furthermore,

these components are maintained at substantially the same relative amplitudes as they possessed in the original signal Wave.

Because the variable frequency segregating means are so controlled that they follow at the analyzing terminal the individual harmonic components, the frequency position of the harmonic components never coincides With the cross-over frequency of two adjacent segregating means. Therefore, the objectionable amplitude and phase modulations which sometimes exist in the fixed band selecting system are avoided.

At the receiving, or synthesizing terminal, the previously mentioned block effect is greatly reduced, or is eliminated since the synthesized signal is composed of harmonically related components that are numerically equal and substantially similar in their relative amplitudes to the components in the original signal wave. These relationships result in the reconstruction of a synthesized signal the envelope of which closely resembles that of the original signal wave, and which contains a greatly reduced number of discontinuities. This characteristic might be otherwise stated to be that the envelope of the synthesized signal varies more smoothly from one to another of its values than is usual in the previously described fixed-band system.

An apparatus for suitably performing the previously described analyzing and synthesizing functions is indicated in block schematic form in Figs. 1 and 2, when arranged with Fig. 1 to the left. The details of various of the circuit arrangements that are utilized in the spectrumamplitude branch of this system are illustrated in Figs. 6, 7 and 9 to Which reference Will be made in the following description.

No speciiic circuit details are shown for pitchdefining circuit 22, since the circuit arrangement that would be employed for this unit Would be governed by the type, or characteristics, of the signal Wave that is to be analyzed. For the successful practice of this invention, it will be satisfactory if the pitch-defining circuit 22 produces a slowly uctuating unidirectional voltage, the amplitude of which is at all times substantially linearly related to the fundamental frequency of the signal Wave. Such a circuit arrangement is disclosed in Fig. 2 of the above-mentioned Dudley Patent 2,151,091, March 21, 1939.

'I'ransmitting and receiving variable selection networks 28 are substantially identical. Their details are shown in Fig. 6, in which bias batteries 373, 32, rectier ad and potentiometer 3B cooperate with a unidirectional pitch-defining voltage from circuit 22 obtained over connection circuit 26, to control the potential of anode 38 of the multigrid electron discharge device or pentode 40. Source 42 supplies anode 38 through the serially connected windings 44, 44', 44 44 of the saturable reactors 46, 4S', 4S 5G11 in variable ilters 2p, 3p, in np. These filters correspond to respective harmonic components of a fundamental component o Incoming message Waves are supplied over connecting circuit 2i! through decoupling resistor 5S,

which is connected in series with the serially connected resonant circuits of the variable filters 2p, 3p, etc., and is preferably several times as large as the resistive component of any parallel resonant combination at its resonant frequency.

Variable filters 2p, 3p, ip np are connected as a series termination for the incoming circuit 2i). Each filter includes a resonant circuit composed of a capacitor 52, 52', 52, etc., in parallel with an inductive element that includes the coil winding 48, 48', d3", etc. Each filter is selective of an individual Wave component of the incoming signal Wave by virtue of the selective properties of its associated parallel resonant combination the frequency of resonance of which is continuously variable between values that correspond to the lowest and highest frequency values that may be assumed by an individual harmonic of the fundamental signal component. Thus, for example, if the fundamental component of the signal might be expected to vary between frequencies of 100 and 300 cycles per second, the parallel resonant circuit of lter 2p would be continuously Variable from about 200 to 600 cycles per second. Filter 3p would be similarly arranged for the third harmonic component covering a range of about 300 to cycles per second, and the other lters would be arranged in like manner.

Since lters 2p, 3p, etc. differ from each other magnitudes 1dof .their circuit compoin the W fees 991.5 i nts, K

' vaf corre-spending' to' fthe c;

The agjrangem'ent of the variable speed enabler n' will ne :ne ,fsf .itil if;

trode of electron discharge device or triode |28. and over connecting circuit |26 to the pulser branch 12.

The puiser branch 1E comprises a number of so-called trigger circuit units or stages of the well-known Eccles-Jordan type. These circuits are serially connected in a chain arrangement in such manner that as pulses are received over connecting circuit It, succeeding ones of the trigger circuits are actuated to deliver a positive voltage enabling pulse on their output circuits |54, |54', etc. Each trigger unit involves on upper and lower electron discharge device or triode |32, |34 or |32', |34', etc. Since each unit is essentially the same as each other unit, only the rst one involving triodes |32, |34 will be described. As in the usual trigger circuit unit, current conduction occurs in only one circuit branch at a time, and these units are arranged such that current initially ows in the upper triode |32 of each unit. Anode power is supplied from battery source |36 through anode resistors |38, |45. Resistors |42, |44 connect the anode of each triode to the control electrode of the other, and grid leak resistors |45, |48 connect to a suitable source of biasing battery. Pulse output circuit |54 is connected to the anode of the upper triode |32. The anode of lower triode |34 is connected to the control electrode of the next succeeding trigger unit through coupling capacitor |50. Capacitor l5!) is selected of such value that, combined with anode resistor I4!) and control grid resistor |48', it possesses a time constant which is substantially twice as long as the duration t of the short sharp negative pulse 99,l which is transmitted from the anode of gas triode '14, through coupling capacitor 98 to triode |00. The opening of switch |52 in connecting circuit IIS, opens the cathode circuit of all of the lower triodes |3il. |34', |34", etc., and forces conduction in the upper triodes |32, |32', |32", etc.

As was previously stated, the arrangement of the variable speed sampler 6|) is substantially identical at the transmitting and receiving stations. Each sampler 6l! contains as many individual sampling, or gating circuits, the details of one of which are illustrated in Fig. 9, as there are variable iilter circuit branches in the selection networks 28. The output pulses from the trigger circuit units of enabler 62 (Fig. 7) are supplied to respective ones of these sampling circuits over connecting circuits |54, |54', |5t", etc. Each connecting circuit |54, |54', |54", is connected through a coupling capacitor |53 to winding |55 of pulse transformer |56 in a respective one of these Fig. 9 circuits. Transformer |56 includes three windings, |55, |57, |59, which are arranged in such manner that when a positive pulse is passed through the center winding |55, the control electrode end of each of the two outside windings |5'i and |59 is made positive with .respect to the cathode end. Two electron discharge devices or triodes |58, |68 are arranged parallel, but with anode-cathode circuits conducting in opposite directions. Coupling capacitors |56, |68 connect the control electrodes of triodes |58, Hit to the respective transformer windings |57, |59. The addition of grid leak resisters |52, |64 converts each triode |58, |52) into a grid leak detector. Values for these component-s are chosen such that the control electrodes oi the triodes are biased below cut-ofi, by grid rectification of the pulses from connecting circuit |55., during the period between 911.15.25. When no positive voltage impulse is actually being received from that circuit. When a positive voltage impulse from enabler (32 is transmitted through capacitor |53 and winding |55, the control electrode of each triode |53, it) is momentarily driven strongly positive and each triode presents a low impedance conduction path between its input circuit Vit and its output circuit |212. At the transmitting end of the system, the input circuit il@ of each sampling circuit is connected to the output of a respective one of the variable iilters 2p, 3p, etc., in the variable selection network 28. At the receiving end (Fig. 2), input circuit |70 ci each sampling branch is connected in parallel to the output of low-pass lter 226 of receiving distributor ii t (Fig. 2). At the transmitting end, output circuit ii'i. cf each sampling branch is connected in parallel to the control electrode T4 of electron discharge device or triode llt of combining circuit llt. At the receiving end (Fig. 2), output circuit H2, H2', W2", etc., of each sampling branch is connected to the input of a respective low-pass lter 23d, 235', 235", etc., where frequency components of the sampling frequency are eliminated.

At the transmitting end, combining circuit |73 (Fig. l) comprises a triode |15 having a control grid electrode i'ill, a storage capacitor itt connected in its control grid-cathode circuit and a cathode load resistor |82. Lownpass lter |84 is connected across load resistor |82. The cut-off frequency of filter |84 will be determined by the number of harmonic components that may exist in the original signal, the number of sampling cycles per second of sampler Sli, and a desired quality of the reproduced signal. For an assumed case where the amplitude modulations of the signal wave may be expected to vary at a syllable rate which will probably not exceed about eight changes per second, the pitch frequency may be expected to vary from about 100 to 300 cycles per second and the signal band width not exceed about 3,000 cycles per second, the cut-off frequency of lter |85 might well be at some value between 250 and 500 cycles per second, depending upon the desired quality of the reproduced signal. i

vConnecting circuit connects the output of filter Ili to transmitting distributor |52. Distributor |92 may be any suitable electrical or mechanical commutator arrangement for distributing the signal derived from lter Idd to a number of relatively low speed outgoing channels. In Fig. 1 this arrangement is symbolically indicated as using a mechanical commutator |94. Circuit itil is connected to wiper arm |38 of mechanical commutator iSd, which has as many segments |55 as there are outgoing channels. Synchronizing arm 255 is mounted on the same shaft as, but is insulated from, wiper arm H38 and, on each rotation, contacts synchronizing segments 262, 2%, the latter of which is supplied energy from positively poled battery 2M. Connecting circuit 65 connects synchronizing segment 252 to the control electrodes of triodes I3 and |29 in the transmitting variable speed enabler t2 (Fig. '7), and delivers a synchronizing pulse to these triodes each time arm 25D bridges contacts 202, 293. Wiper arms |98, 250 of commutator 59|! make one complete revolution during the time required for variable speed sampler 5t to sample the output of each variable lter of variable selection network 2S. Where, as here, the out-oil frequency of each low-pass lterv58 in the trttnSmitting variable selection network 2'8 is about "puts of the shaping combinedinfalsinglecircuit 242 to constitute the i cycles per second'i'and' commutator ISF makes a complete revolution-TT in -1`/f=,0`of fa second, 'there should be provided-'atleast te'n segments "l 9E and 'tenffoutgoing chal-inelsifin" order 'to 'secure 50G samples of" the L'oiitliutof filter I 813 each second.

"Each segment-'196 iis" connectedto' 'ia respective I electron dischargefdevice or-triode Ztj", etc;,

Whiclillherefsynibolically` indicates any suitable 'frequency modulated oscillation source; the freuuen'cyf-of' oscillation of which is'variablefi-n' accda'ce with the potential 's'toredon'z storage capacitor 208, 20S', etc., in the control ugrid 'cathode circuitV ofeach oscilla'ton- ,A1 cathode re- "Sisto-"'ZlI-ZIBZZHVJ, `etc.f,` i's'lin'cluded inthe 'cathode circuit-idf eachoscillator Ztofprovide a fp'roper' impedance "transformation between the 'oscillatory source and the respective outgoing channel.l ""'f UY* x t'. fst-c1 r 3 :a 'At th receiving nd (Figi.` 2li, receivingdistribor 218 7' n ielectrica'laori mechanical 'commutator222f1wlich is substantially the coun- `-t'eriart *of 'transniitting-"fIcmmutator '1194. The ft'pitif f i'equency'-modulation detectorsy 22u, '2 2 0 f 22 U etc.;fare- AA'connect'ed.'to2- the' `respective segments of commutatorllZZiwhichihas.a wiper Jarirrr 224; a'-s'ynchroi'zing"*mmh-223::and synch'rnizin'g' 'segments 225,! 221,y vthe :function of ivhielr is the same as tlatof the 'flikelitemsz in 1the transmitting commutation Connecting `cir- 'cuit l232 fconnectssynchronizing segment 225 to tli'receiving variable'speed enahlerfSZf. 1 A'I'heldetails' ofA variable ,'speed;A 'sampler 6c' and "variablef'speedenabler S2-are the same f as Ithose explained in connection with the transmitting f-unjitid asfwere'-discussed in detail in connection with Figs -7f'andi9s1iiii@- i The'lsour'ce' of synthesizing energyf234. may1be any conventional,zarrangeinenivfsuch;l for examplef as? `v'vas`disclosed?V in the v:previously mentioned Dudley patent in which'l aY relaxation oscil- "la'tor "provided fthe P voic'ewffenergy and a noise generator provided'thev'funvoiced-or,=hiss type "of energy. Receiving variablefse'le'ction network f"2-8-comprise's the fsame: arrangement 'fof variable '/iltrs l 217,' 3p; "'Ap,etcl;, ias' was explained infconnectin lWith' Figlfi inJ vthe :transmitting fapparaftus. ModulatorZBB includesfa shaping network fSNzifSNai; lSNi etc.,'rcorrespondingf' to .f each .harmon-ic componenti'- Theseshaping networksmay 'b'elany conventionaland convenientitypeof am- "plitude modulator, ffsuch -1 as an amplifier, vfthe 'gaiir'y ofwhich' is4 controlled i-byi'a unidirectional synthesizedsignal Toutput? s """Th'e' m'annr-'in=Whichfthesystem of this inventionfoperates' mayfbe best understood from va "description-"of its "analyzing and vsynthesizing -'functions' when considered with general--ref erence to the transmitting and receiving equipc 3125122521," .i' iL.; "S21 'mentiarrangementstindicated irl-Fig .v

with occasional specific references tofthezrother gurescofzthe; drawing. For fthe purpose; of;this :descriptiomzt will :be assumedxrthat thelwave ztbezanalyzed and'ssynthesized .is'a vspeech 4sigmaigre/amen comprising so-,called-`= voicedi and `funvoicedf rintervals;` During theI voicedi--inaterval, the'r-signalz energy is'.Z grouped in 12a num-` ficiei'ffzoff Wave:'components :that are fdistributed :in fharmonic "relatiomto afundamentalicomponent that mayzor;mayfnotf-.actually be presentrinpthe -Waver lDuringr,thevunvoiced; interval, :the sig- :nalv energv-isdistributedimore or less continuiouslythroughout the frequency spectrum of the SigI'IalfbaIld. 1;,2 :i 4zf if fsf zum s, (L ,:*"The pitch offsuch a-signal is ,variab1e at;a relr'atively llow rate Whiclndoes` notordinarily,ex,F ceedabout;eightzchanges inia second, and which has l.-generallyr;beenutermed' the-fsyllahicr rate. The :amplitudes,notI rthefsignal` fwave 1 components :arcta-lso variableyfand imay change;-at=.the same #relatively low. syllabicrate not: to iexceediabout eight changessinnaisecondx T-,hezrangeofsthe `pitclnLorifundamental frequency,=of such assignal might Well begfrom about 100 cyclesperivsecafzcndatoBOO cycles-per second, and the-maximum A'bandwidthr of such sa, 4signal :might 4be .fconflned betweenaxed frequency limits, :for example, 200 :to l:3200. cycles jper second.t -From the foregoing ,fit y is t evident that 4the Ynumber-fof Wave compafnentslthatmayhe-present inthe signal waveat any instantie determined A-by :tha-pitch, ,or .,-fref ,quency of thewfundamentalfcomponent. lfhis conditionsisrindicatedfin the idealized energy (distribution. .representations lof Figs. BzzfandA. .nuFig'..-z3. thereliswi-ndicated the distribution of .signaii energy,.'across the Lfrequency band foin a :low pitchedizoice. ,Fignfl -indicatesthisdistribution for azliigh:1 pitchedlvoicez Thesolid.ver. `tical lines in these .Kiguresi represent =the `relative amplitudes of individual harmonic Wave componentssof thefnndamental frequency. .Thesisnicanceaof tthe bloclnareasVv that, are defined hy4 the solidand dotted lines offthese figures will ,be,.explained. later. VA,A continuous curve ,connecting the .top ofeach individualicomponent `,would A represent the amplitude characteristic of thessignals frequency spectrum :at the giveninstant-,l-and might be termed the signals .spectrumvlarnplitudecurve.. ESuch a curve is-.illustratedin Figmrsfhis -.curve. 188, .representsu ar voltage distribution which ,lcontainsiall :ofgthe essential .information regarding fthe .relative amplitudes .of :the-:Waves componentS,-.and itY may 4beaused ito y controlv the amplitudesfof Wave components .frm/nza. separate source :of4 energyf-to reconstruct. -.or synthesize; that portionfiof-,the,original signal wave. The frequency, or .spectrum location nfE these-Wave'components is controlled by the pitch-lof.thezsignal` r;=;From the foregoing "it is evident. Vthat asfthe numbers-off, harmonic-ywavel components ,varies -from--instant1 to instantfthe numberfof-yfpoints ron. the :continuous V envelope; f or;:;spectrumam .plitude'zcurva which aref representative of the amplitudesnf theindividual components, is sim- ;ilarly1variable. If, then, the synthesized-signal is toL-be a faithfulzreflection of the relative am- :plitude of eachy harmonic' component Kin the `original-- signalwave,-a variable number of `har-- -monic amplituder measurements must tbe made #during succeeding intervals in analyzing the originalsignal wave. 'f s "u Nowassumerthat `a voiced f' signal Wave is supplied over input circuit 20 from a source ciated capacitor 52,

that is vnot shown. This wave is divided between the pitch-delining circuit 22 and the transmitting variable selection network 28. Pitch-defining circuit 22 produces a slowly fluctuating unidirectional voltage, the magnitude of which varies in direct linear relation to the frequency of the fundamental component, or pitch, of the signal. This voltage output appears on the three connecting circuits 24, 26 and 64, and is available for control-uses which will be described.

rEhe voltage on circuit 2t controls the frequency setting of each of the variable filters 2p, 322, etc. in transmitting selection network 28. Refer now to Fig. 6, in which the details of these Variable filters are illustrated. With no voltage on circuit 26, bias battery 32 maintains the anode current of pentode 4l! at some minimum value, the magnitude of which is sufficient to cause each inductor 48, 43', fit |38 to resonate with its asso- 5t etc. at a frequency that is equal to the minimum value that its respective Wave component may assume. Thus, for filter 3p in our assumed example, this minimum resonant value would be 380 cycles per second, and the effective induotance of inductor 113' would be regulated to a suitable value to resonate with capa-citor 52 at this value. As the voltage on circuit 2S, with polarity as indicated, exceeds a minimum value corresponding to 100 cycles per second, the current ow in potentiometer 3S causes a voltage drop, which when added to the potential of battery 32 increases the anode current of tube dil. This increased anode current increases the flux density in the core of each reactor Alli, lit', etc., and reduces the eiective inductance of each associated inductor di), t8', 12.8", etc., to a new value such that it and its associated capacitor 5i?, E52', 52, etc., resonate at a frequency correspond'mgT to the respective harmonic ci the signals instant fundamental frequency. The resonating elements of each iilter 2p, 3p, etc., are connected in series, and together with the coupling resistor 5&3 form a termination for the lower halr" of input circuit Ztl. Signal energy corresponding to the resonant frequency of the respective variable filter oscillates between the capacitive and inductive branches of each resonant network, and induces in the respective secondary winding, 59, 5t', 59, etc., a corresponding voltage which appears across the open termil nais of that winding. These voltages regulate the respective control grid cathode potentials of the isolating triodes 54, 5d', ctc., and produce their replica in the respective anode circuit of the triode. anode voltages are rectiiied and ltered in the usual manner in circuits 56, 5t', E8, etc. These actions produce on each connecting circuit lll), l'ii, HG lill, slowly fluctuating unidirectional voltages, the amplitude of each of which is representative of the amount of signal energy that is segregated by the respective variable lter 2p, 3p, etc. The numbers, locations and amplitudes of these fluctuating voltages will vary as the frequencies and amplitudes of the harmonic wave components change in the signal wave.

These unidirectional voltages are supplied over connecting circuits llt, I'l, etc., to the transmitting variable speed sampler t@ where each one is sampled 50 times a second. It will be recalled that the signal energy distribution during any instant reflects the number of harmonic components that are present at that instant, and also that the synthesized'signal is to be composed of the proper number of harmonic components having correct The alternating components of these l A14 relative amplitudes with the proper spacing between the harmonics. In order to meet these requirements, sampler til varies the interval between successive samples from one sampling cycle to another in such manner that during low pitch intervals of the signal it takes more samples during each cycle than are taken during intervals of higher pitch. Not only does it take more samples during the low pitched interval but the interval between successive samples is shorter than the intervals between successive samples during the high pitched signal interval. The time relation- Ship between these samples closely approximates the relative frequency positions of the actual harmorne components durmg low and high p1tch signal intervals.

Each connecting circuit Fill, Nil', ll, etc., connects the relatively large capacitor (not shown) in the output of its low-pass iilter 58, 5B', 5t", etc., to the input of a respective individual gating, or sampling circuit, the details of which are illustrated in Fig. 9. Referring to that figure, when a positive voltage impulse is received over connecting pulse circuit 151i, the anode-cathode paths vof the oppositely conducting triodes |58 and ist momentarily provide low impedance conduction paths between the input and output circuits lill, Il', respectively. Therefore, as each sampling circuit is momentarily made conductive, the charge that is stored in its associated low-pass lter circuit is quickly transferred to the relatively small contrcl grid storage capacitor |30 in combining circuit llil of Fig. l. The gating, or sampling circuits contained in sampler tu are sequentially operated, or made conductive by pulses received over the respective connecting ciry cuit lifl from the respective trigger circuit units in the pulser branch 'i2 of variable speed enabler E2, and a new sampling cycle is started each 1%30 5 of a second. The timing of each sampling cycle as well as the regulation of the interval between successive sampling operations in the same cycle is controlled by the operation of the transmitting 'variable speed enabler t2, the details of which are illustrated in Fig. 7.

The manner in which the variable speed er1- abler ti.; operates to successively activate the sampling, or gating circuits in sampler G0 may be best understood by reference to Fig. 7. Consider first the operation of this circuit as it regulates the interval between successive samples in any sampling cycle of transmitting variable speed sampler tu. Pitch-defining unidirectional voltage from pitch circuit Z2 (Fig. l) is received on connecting circuit', with the polarity as indicated. When no voltage, or when the minimum voltage, is received from pitch circuit 22, the variable relaxation oscillator branch 68 produces a maximum number of short, sharp, negative voltage impulses 9S, each of duration t Under these circumstances sampler 6&3 takes a maximum number of samples during each sampling cycle. It might be Well to note at this time that this condition only exists during unvoiced intervals of the signal, or during voiced signal intel'- vals when the fundamental frequency is at its lowest limit. At such times, each variable lter 2p, 3p, etc., of transmitting selection network 23 is at its minimum frequency value, and energy is selected from the signal wave at a maximum number of frequencies. If the signal has its minimum fundamental frequency at this time, eachilter will select the corresponding harmonic wave component.` If the signal is in an unvoiced3 interval at this time. each lter selects a:representatiye.` amount of l energy atits resonant frequency. In eitherevent, the spectrum-amplitudecha-racteristicrof the signal'wave `is `determined at, a `maximum number of frequencies.

Returningto the-oscillator branch 58 (Fig. 7) the frequency of recurrence of pulses=99 is con trolled -by the potential ofV control --electrode82 ofgas-lled triode 14. This potential is in rturn controlled bythe combination of the potentials of `anode 95 of triode '86 and bias battery 95. When no Vvoltage is lbeing received from circuit G4, `anode 95 and control electrode 32 are at their maximum potentials. When there is Vpresent on circuit-@4. apitch-deiining voltage that -exceeds the., potential of bias 4battery E8, current ows through rectifier 92 and potentiometer S4 to produce a positive grid bias that causes the potentialrofranode 95 to be lowered. This decrease lowers the potential of control 4electrode 82, thereby reducing the .iiring rfrequency of triode 74;.and .the frequency of recurrenceof negative pulses 99.

Pulses Bifrom gas triode 14 inoscillator branch 468 `are .inverted in triode |00, andare coupled throughcathode follower |06. to the control electrode. .|.|.l of cathode follower ||.2. ResistorA H4 is=includedin .the common cathode circuit |.IE of lower triodes |34, |3|, etc., of `the trigger circuit units of pulserbranch '52. Each time that a positive.voltage impulse appears across resistor lill, these cathodes ofthe lower triodes |34, |342, etc., are made sufliciently positive with respect to .their control grids to stop .conduction in any ofV the lower tubes. This pulser branch 12 is arranged such thatall of ,the upper triodes |32, |32', etc., are initially conductive, and therefore, until conduction is started in a lower triode |34 these cathode control pulses are .ineifective The manner inwhich conduction is changed from upper triode |32 to lower triode |313 will shortly be described in connection with Vthe discussion of the manner in which enabler` 62 controls the timing of each cycle of variable speed sampler 6i). .Assume for the .moment that in some manner triode |32 is made non-conductive and triode |34 is made conductive. The anode voltage of triodev |32 rises sharply and a short positive voltage pulse is transmitted over conductor I5!! .to the sampling circuit branch (Fig. 9) of transmitting variable speed sampler 6! that is associated with variable filter` 2p, in transmitting variable selection network 28. 'During the shortintervall of this pulse the sampling circuit (Fig. 9) provides a low impedance pathbetween variablefllter 2p andcombining circuitV |18, in the mannerthat hasbeen described.

When the next pulse 99 is received.` from gas triode 14, the cathode of triode 34 is momentarily 'elevated to Va point where conduction is stopped inthis tube, and' is started in .upper triode |32. The anode potential of triode |34. rises sharply and a positive voltage pulse is transmitted through coupling` capacitor |56 to start conductionin, or to trigger, triode |34. ,This action stopsconduction in triode |32' and .produces on .its connectingjcircuit' |54' a positive voltageimpulse which actuates the sampling circuit branch (Fig. 9) of variable speed sampler 60' (Fig. 1) that is associated. with variable filter` 3p Ain transmitting variable selection network 28. In this. manner, as successive impulses 99 `are received` from the relaxation oscillator .branchl E8 of venabler B2 ,(Eig. .7) ,the successive stages ofzpu-,lser branch 12 areactuated, and .the samplinggcrcuit branches of-sampler r 6 0- (Eiga 1) are 'successivelynactuated toficonnect; their .respective lfilter-branch fof- -variable-selectionl network `2t v.to the'v combining circuit `ifi' 8. Thissuccessive st eppingaction offpulser branch l2' will continue until the last trigger circuit stage-n is operated, :or until current conduction-is vsimultaneously stoppedin all of the lower.; triodes 131|,y |34', etc.,-and` is restored in allxof'theupper triodes |32, |32' .to resetthe puiser branch i 2 for a new sampling cycle. .The manner ,inwhichthisis accomplished will now be explained.

Ai'lhe ,xed duration sampling cycles of sampler ilpareitimed by synchronizing impulses received over connecting circuit 5t. In this described embodiment these' impulses are received from transmrtting distributor; |92 (Fig. l), though this is not a necessary condition for the successful practice of this invention, since these pulses might be obtained from any suitable periodic element that may be operated in synchronism with a similar element at the synthesizing location. The impulse, which arises when synchronizing arm 200 of commutator |94 bridges segments 202v and 263, is substantiallysquare-topped, and is preferably of duration about four times that of time t of pulses S9. The pulse is simultaneously suppliedy to the controlelectrodes of triodes |18 and |29 (Fig. 7) to produce duplicate 4voltage pulses across cathode resistors H3, |2| and IM. The pulse produced across resistor ||4 elevates the cathodes of the lower triodes- |33, |34', etc., and4 `stops conduction' `therein in the previously described manner. rihe duration of this pulse is long, as compared to the time constant .of the coupling capacitors |56, |5l, etc., and their associated resistors, and any residual charge on these capacitors is dissipated during the.` pulse duration. In this manner, theupper triodes |32, |32', etc., of puiser branch '|2 are vrestored to their initial current conduction state, and the pulser branchf|2 is reset for a new .sampling cycle.

During this same time, a duplicate pulse is produced across the cathode Vresistor ||9 of triode H8, which pulse is differentiated intransformer |22. 'Ihe positive voltage impulse product of this differentiation is clipped by rectifier |24 in such fashiony that a single, sharp, negative voltage impulse, as indicated by wave form |25, ensues; this pulse. is. used to start each sampling' cycle, and also to reset the relaxation oscillator branch 68 in such manner that it starts a new timing interval at the start of each new sampling cycle.

The cycle-starting function of this negative voltage impulse |25 is accomplished whenV one portion of it is Vtransmitted throughy coupling capacitorV |43 to the control grid of upper triode 32 in pulser branch l2, .to .cause this4 tube to cease conduction. As waspreviously explained, this causes a potential rise at the anode of. this tube which rise is used to actuate the sampling circuit associated with the first variable filterV 2p in variable selection network28. It also causes the` lower triode |34 to start conduction, and

- readies the chain of trigger circuit units for sequential operation by subsequent pulses received from the gas triode 'M in the oscillator branch 68 of the circuit.

The second portion of this negative pulse |25 is inverted in triode |28, and is supplied over connecting circuit |30 to actuate gas triodel'M andreset the charging cycleof capacitor 16v coincidentfwith the startV of the newsampline' cycle. This action isiindicated by the wave forms 0f Figs. .8A and 8B. i Fig.v BAllustrates the build-'up .and

decay of the voltage across capacitor 76. During the interval "a-b this voltage increases at an exponential rate that is determined by the values of capacitor 76, resistor 18 and battery 80. Triode 'M lires at time "b at a voltage Value that is determined by the potential of its control electrode 82, which value is related to the signals pitch, or frequency of its fundamental component, and the charging operation is restarted. At time 0, coincident with the formation of negative voltage impulse |25 and the start of a new sampling cycle, the positive voltage pulse that is supplied over connecting circuit |30 (Fig. 7) from triode |28 to the control grid 82 of gas triode 74 causes this latter tube to re, and starts capacitor 76 on a new charging cycle. This new charging cycle is indicated by the interval c-d at the start of the new sampling cycle. Coincident with these times a, b, c and d, short, sharp, negative voltage impulses 99 (Fig. 8B) are delivered through coupling capacitor 98 to actuate the pulser circuit 72, and advance the sampling operation in the manner that has been described.

In this manner the enabling impulses on the connecting circuits |52, |54', etc., are advanced from one sampling, or gate circuit to another in the sampler 60, as subsequent pulses 99 are received from the oscillator branch 68 of the variable speed enabler 62. This process continues until, at the next succeeding time 0, the synchronizing pulse from transmitting distributor |92 restarts the sampling cycle, and resets the operation of the oscillator circuit branch 68,

in the manner that has been described. If during this next sampling interval, the fundamental frequency of the signal wave has changed from its prior Value, the voltage on connecting circuit 8d will be changed and. the relaxation oscillator circuit branch 88 will accordingly change its pulse repetition rate. Pulser circuit 72 will deliver enabling impulses over its connecting circuits lfl, |54', etc., at a revised rate, and sampler 60 will sample a different total number of variable lter branches in variable selection network 28 during the next succeeding sampling cycle.

As the sampling operation proceeds, a low impedance path is momentarily provided between the output capacitor (not shown) of each variable lter 2p, 3p, 28 and the storage capacitor |80 in combining circuit |78 (Fig. l). Since capacitor |80 is small relative to these filter capacitors, any potential difference between the two is quickly equalized, and capacitor I 8l) quickly assumes a charge which is substantiallyequal to the original charge on the filter capacitor. When the effect of the enabling pulse is removed, this charge is isolated on capacitor |88 in the grid circuit of triode |76, since the control electrode |74 never becomes positive with respect to its cathode, and the return path through triode |53 (Fig. 9) to the filter capacitor is opened; and there is produced across cathode resistor 82 a potential that remains substantially constant until the next succeeding variable filter in selection network 2B is sampled and a new charge is established on capacitor |88. Thus there appears across resistor |82 a continuous series of voltage impulses such as are indicated by the rectangular block areas that are dened by the solid and dotted lines of Figs. 3 and 4. These areas in Fig. 3 show this condition for a low pitched signal, and the areas of Fig. 4 show the same for a high pitched signal. This fluctuating potential is smoothed in lter |84 etc., in the selection network l to produce a continuous spectrum-amplitude voltage which is similar to the original spectrumamplitude curve |88 of Fig 5, and in which'the amplitude at any instant is representative of the amplitude of the harmonic component that is being sampled at that instant. This smoothed voltage curve at the output of filter |84 is a continuously variable function which may undergo a complete transformation fty times each second, if the frequencies and/or the relative amplitudes of the components in the signal wave change with that rapidity.

This variable amplitude unidirectional wave, when used in combination with the pitch-defining voltage on interconnecting circuit 24, may be used to directly control the operation of the synthesizing apparatus. However, in an embodiment such as is being described, it is usually desirable to transmit these control voltages a considerable distance. This transmission may be accomplished by an inverse time division multiplex arrangement whereby the spectrum-amplitude voltage wave may be distributed to a plurality of low speed, or narrow bandwidth, outgoing channels in each of which the duration of the distributed signal may be considerably increased over its instantaneous value. The combined spectrum-amplitude wave is transmitted over interconnecting circuit 90 to transmitting distributor |92. As wiper arm 200 bridges the synchronizing contacts 282, 283, the previously mentioned positive Voltage synchronizing impulse is transmitted from battery 28d over interconnecting circuit 66 to the variable speed enabler 62 where it starts a new sampling cycle, and resets the phase of oscillation in the relaxation oscillator circuit branch 68 in the previously described manner. Wiper arm |98 contacts each commutator segment |96, and stores on its associated grid capacitor 288, 288', 288, etc., a potential corresponding to the instantaneous voltage value of the spectrum-amplitude curve as it appears at the output of filter |84. Each frequency modulated oscillator 286, 286', etc., then transmits a distinctive frequency that is individual to the potential stored on its grid capacitor 20B, 208', etc., until a different potential is stored on this capacitor during the next rotation of arm |93. These frequency modulated oscillatory waves are transmitted to the receiving, or synthesizing station (Fig. 2) where they are received, demodulated and used to control the operation of the synthesizing apparatus in a manner which will now be described.

Referring to Fig. 2, the incoming frequency modulated waves are received by receivingdistributor 2|8 where they are demodulated in frequency modulation detectors 228, 220', 228"etc., and the resulting unidirectional voltages are applied to the commutator segments of the receiving commutator, which is again symbolically indicated by a mechanical commutator 222. Transmitting and receiving distributors |92, 2|8 may be synchronized in anysuitable manner in which several are well known in the telegraph art.

Wiper arm 223, in conjunction with synchronizing segments 225, 227 and the associated battery, produces on interconnecting circuit 232 a positive voltage impulse in the same manner and at. the same relative time as its counterpart is produced at the transmitting station; This impulse'- is usedto start each sampling cycle of the receiving variable speed sampler 60, and to reset the phase of the relaxation oscillator circuit branch ,63 'in the receiving variable speed enabler 62 in the same Vtransmitting end counterparts of these circuits.

In describing the operation of this receiving apparatus, the same reference numerals are used to indicate circuit components as were used at the transmitting station, where the circuit components are identical in nature.

The time-divided spectrum-amplitude wave is reassembled by the action of commutator 222 and low-pass filter 226. At the output of low-pass illter 226, this voltage curve has substantially its original form which is also substantially the same `as the original spectrum-amplitude curve as indicated by curve 1'88 of Fig. 5. This voltage is supplied over circuit 11! to the receiving variable speed sampler 60 which, as in the case of the transmitting apparatus, comprises a set of circuits in accordance with Fig. 9 for each harmonic component that may be expected in the original signal wave. In other words, there are as many receiving sampling circuits in accordance with Fig. 9 as there are variable filter circuits 2p, 3p, etc., at the transmitting, or at the receiving end. Each sampling circuit is sequentially operated by an enabling voltage pulse derived over interconnecting circuits (54, 854', |54", etc., from the receiving variable speed enabler E52. This enabler I62 is substantially identical to the transmitting variable speed enabler 2, and always operates at the speed, or frequency of that enabler, since its frequency of operation is controlled by the pitchdefining voltage received over interconnecting circuits 24, 64 from the transmitting end pitchdenlng circuit 22. In this manner, each sampling branch circuit of receiving sampler @il sainples the reconstructed spectrum-amplitude voltage at a time when its amplitude is indicative of the amplitude of a designated harmonic component in the original signal wave. These sampled voltages are applied to the modulator 23B, where each individual sampling branch circuit is associated with an individual shaping network SN2, SN3, SN4, etc.

The source of synthesizing energy 234 comprises a source of buzz, source of hiss, or unvoiced energy. The fundamental frequency of the voiced energy is controlled by the amplitude of the pitch-denng voltage on circuit 24, and thus is substantially the same as the signals fundamental frequency. This energy is supplied over connecting circuit 235 to the receiving variable selection network 28, the character and the control of which by the pitchdefining voltage obtained over connecting circuits 24, 26 are identical with the character and operation of the like apparatus described at the transmitting end, and the details of which are illustrated in Fig. 6. Variable filters 2p, 3p, 4p, each have resonant frequencies which correspond to adjacently-related harmonic components of the fundamental frequency of the buzz or voiced energy received from source 234. These filters operate to segregate the energy from source 234 into individual harmoncally-related frequency components which are then supplied over yconnecting circuits 24D, 240', etc., to each of the shaping networks SNz, SNs, SN4, etc. where they are combined with the individual amplitude control voltages that are segregated by the sampling branch circuits of the receiving Variable speed sampler Ell. The amplitude of the harmonic component is modulated in each shaping network in accordance with the amplitude of the segregated control voltage, and in this way is made to simulate the amplitude of the harmonic component of or voiced energy, and a i the same frequency in the original signal wave. The outputs of the shaping networks are combined in the signal output circuit 242 tc constitute the reconstructed, or synthesized signal.

In this preceding explanation it has been assumed that the signal wave comprised only voiced energy, because in this type of signal both the amplitudes and the frequencies of the harmonic components are variable. During those intervals when the signal wave comprises only unvoiced energy, the pitch-defining circuit 22 produces no output voltage, and the variable filters 2p, 3p, 4p, etc., in the transmittingand receiving variable selection networks 28 are adjusted to i correspond to the lowest frequency that their respective harmonic component may assume. Under these conditions each transmitting variable filter 2p, 3p, etc., selects a portion of the unvoiced signal energy, and there is produced a spectrumamplitude voltage Wave which is composed of the maximum number of energy samples. This is desirable since, in the unvoiced type of signaLthe energy is distributed in a continuous fashion throughout the signal spectrum instead of in harmonic groupings as in the case of the voiced signal. This spectrum-amplitude wave is transmitted to the receiving, or synthesizing end of the system, in the previously described manner, and produces at the output of the receiving variable speed sampler L'i, a series of control voltages the amplitude of each of which is indicative of the energy in the respective portion of the signal at the given instant. At the same time the variable lters 2p, 3p, etc., of receiving variable selection network 28 are adjusted to harmonic frequencies of the same minimum fundamenta-l frequency, and similarly select the maximum of samples of unvoiced, or his energy from the source of synthesizing energy 234. This selected noise energy is supplied to the individual shaping networks lSNz, SNs, SN4, etc., where it is modulated in accordance with the amplitude of the associated control voltage in the previously described manner to produce the synthesized unvoiced signal in output circuit 242.

In the system Athat has been described, the variable frequency filters 2p, 3p, 4p, etc., of the transmitting and receiving variable selection networks 2B comprise a series array of parallel resonant circuits, each of which selects a harmonic component the frequency of which corresponds to its resonant frequency. In Figs. 10a, 10b, and ll, there are shown variable filter network structures which may be employed in place of the serially-connected parallel resonant structures illustrated in Fig. 6.

Fig. 10a. shows the conventional mid-shunt band-pass filter, and Fig. 10b shows the equivalent mid-series band-pass filter structure in which the series and shunt inductive members are made variable. These inductive members` may comprise saturable core reactors such as were used in the variable filters described in connection with Fig. 6. If the value of the series inductive member L1 is varied substantially inversely as the lower frequency limit of the pass band is to be varied, and if the shunt inductances L2 are varied substantially inversely as the square of the lower frequency limit of the pass band, the characteristic impedances and the pass bands of the filters remain substantially constant, but the frequency location of the pass bands is varied in accordance with the changes in the inductive members of the filter.

Referring to the alter' structur -of rig. 1'1, 'it

vwill be noted that `all four reactance elements are made variable. Inductor L, and capacitor C3 form the series branch impedance elements, and inductors L4 and capacitor C4 form the shunt branch impedance elements. Variable capacitances for this purpose may be obtained from conventional reactance tube circuits, or from ceramic condensers of the titanate class, the alternating-current capacitance or which may he varied by a superposed direct voltage, as is described in the copending application of A. M. Curtis, Serial No. 704,151, filed October 14, 1946. In this structure, when all four reactance elements are simultaneously varied, the geometric mean between the upper and lower frequency limits of the pass band varies in inverse relationship to the changes in the reactance elements. From this it follows that the bandwidth of this filter is not constant but is proportional to the frequency location of its pass band. In some embodiments of the invention this may be a useful characteristic to employ since the spacing between adjacent harmonics is proportional to the frequency of the fundamental component of the wave, and hence is proportional to the frequency location of each specific order of the harmonic components.

Although in the foregoing description, the invention has been described with particular emphasis upon its incorporation in a vocoder type of transmission system wherein speech signals are transmitted with reduced frequency range, and in which specific circuits parameters are recited, it should be realized that its scope is not limited to such an arrangement. It is evident that the invention has many applications in the field of wave analysis in cases where it may be desired to obtain a rapid, continuous and accurate indication of the magnitudes of the various harmonic components of a wave of variable fundamental frequency. Other desirable aspects of the invention will undoubtedly occur to those skilled in the art, and suggest embodiments thereof which do not depart from the spirit and the scope of this invention.

What is claimed is:

1. A system for communicating a message that is represented by a frequency band of waves, which system comprises a source of waves having a discrete energy spectrum, said source including means for segregating said energy into a plurality of harmonically related wave components corresponding in frequency but dependent in amplitude relative to the harmonic overtones of the first-mentioned wave, analyzing circuits the attenuation frequency characteristics of which are variable in accordance with the fundamental frequency of said message wave for determining the syllabic time rate of change of energy in each significant harmonically related component of said message wave, and means controlled by the products of said determination for imparting substantially the same syllabic time rate of change of energy to corresponding harmonically related wave components from said source.

2. lThe method of operating on a variable frequency complex signal wave that is representative of a message signal, which comprises dividing said wave into a first and a second portion, determining the fundamental frequency of said first wave portion, individu-ally segregating from said second portion a plurality of harmonically related wave components, and varying the nurnbfber of said individually segregated components in accordance with changes in said fundamental frequency determination.

3. The method of operating on a variable frequency complex signal wave that is representative of a message signal, which comprises dividing` said wave into a first and a second portion, determining the fundamental frequency 0f said first wave portion, individually segregating from said second portion a plurality of harmonically related wave components, and varying the number of said individually segregated wave components in inverse relation to changes in said fundamental frequency determination. i

4. The method of operating on a signal wave that is represented by a quasi-periodic complex wave the frequency of which varies at a syllabic rate, which comprises dividing said wave into two portions, deriving from said first portion a representation of the fundamental frequency of said wave, continuously selectively segregating from the second portion individual wave components representative of each individual significant harmonic overtone of said fundamental frequency, varying the number of said individually segregated wave components in accordance with changes in the derived fundamental frequency representation, repetitively deriving an indication of the magnitude of each of said individually segregated wave components during uniformly recurring time intervalsfand combining said magnitude indications into a single variable representation the magnitude of which is at any instant representative of the magnitude of an individual one of the harmonically related wave components in said quasi-periodic complex signal wave during a designated interval.

5. The method of operating on a variable frequency complex signal wave the frequency of which varies at a syllabic rate which comprises, dividing said wave into a rst and second portion, analyzing said first wave portion to determine the fundamental frequency of said wave, individually segregating each from the other each harmonically related wave component in said second wave portion, repetitively deriving at least twice during each syllabic interval an indication of the magnitude of each of said individually segregated harmonically related wave components and combining said magnitude indications into a single representation the variable magnitude of which is at any instant representative of the magnitude of an individual one of the harmonically related wave components in said complex signal wave during a designated interval.

6. A signal analyzing and synthesizing system which comprises a plurality of wave segregating means each of which is responsive to an individual one of harmonically related wave components that represent a signal wave, a plurality of analyzers for separately determining the syllabic time rate of change of energy in each of said segregated components, a source of complex Waves having its fundamental frequency equal to that of said signal wave and having the relative amplitudes of its components independent of the relative amplitudes of the comporients of said signal wave, a plurality of selective circuits corresponding to said analyzers which are variable in accordance with frequency changes in said signal wave for selecting individual harmonically related components of said complex wave, a plurality of modulators individually responsive to the time rate of change of energy determinations received from respective ones of said selective circuits for controlling the 23 rate of change of energy in the individually segregated components of said complex Wave, and means for combining said modulated components.

'7. A. system for analyzing a variable frequency complex signal Wave which comprises, means for deriving from a first portion of said wave an indication of the fundamental frequency of said wave, a plurality of frequency sensitive networks including reactances for individually segregating from a second portion of said wave each harmonic component contained therein, means for varying the reactive values of said network elements in accordance with changes in said derived fundamental frequency indication, and means for deriving an indication respective each Wave component of the energy content thereof.

8. A system for analyzing a variable frequency complex signal wave which comprises, means for deriving from a first portion of said wave an indication of its fundamental frequency, means for individually segregating from a second portion of said Wave each individual harmonic component contained therein, said means comprising a plurality of variable frequency-sensitive networks including reactances, means for varying the reactive values of said network elements in accordance with changes in said derived indication, means for deriving an indication respective each segregated wave component of the energy content thereof, and means for combining said respective energy indications into a single indication the characteristics of which vary from instant to instant in accordance with the energy contents of the segregated components.

9. A system for analyzing a variable frequency complex signal Wave which comprises means for deriving from said wave an indication of its fundamental frequency, selective means responsive to said derived indication for segregating.

into individual channels each significant harmonically related component in said signal wave, said selective means including a plurality of reactive impedance elements the reactances of which are responsive to changes in said derived frequency indication, means for sampling the energy content of each segregated component during recurring uniform intervals, and means for combining said samples into a single variable representation the amplitude of which at any designated interval is indicative of the energy content of a segregated component,

l0. A system for analyzing a complex signal wave the frequency of which is variable at a syllabic rate which comprises, means for deriving from said Wave an indication of its fundamental frequency, segregating means responsive to said derived indication for segregating into individual channels each significant harmonically related component in said signal wave, said segregating means including a plurality of impedance elements having reactances which are responsive to changes in said derived frequency indication, means for sampling for a predetermined minimum period the energy content of,

each segregated Wave component during each syllabic interval, means responsive to said derived frequency indication for varying the sampling rate in accordance with changes in said indication, and means for combining said samples into a single variable representation the amplitude of which at any designated instant is indicative of the energy content in a respective -one of said individual channels during said sampling interval.

11. A system for analyzing aA complexsignal wave, the frequency of which is variable at a syllabic rate which comprises, means for deriving from said wave an indication of its fundamental frequency, segregating means responsive to said derived indication for segregating into individual channels each significant harmonically related component in said signal Wave, said segregating means including a plurality of impedance elements having reactances which are responsive to changes in said derived frequency indication, means for sampling for a predetermined minimum interval the energy content of each of said segregated harmonically related wave components, means responsive to said derived frequency indication for controlling the sampling rate during each of said sampling periods, means for repeating said sampling operation during recurring equal time intervals, and means for combining said samples into a single variable representation the amplitude of which at any designated interval is indicative of the energy content in a respective one of said harmonically related Wave components.

12. In a system. for analyzing a complex signal Wave the frequency of which is substantially constant for short intervals of variable duraation, means for deriving from said Wave an indication of its fundamental frequency during each interval, segregating means responsive to said derived indication for segregating into individual channels each significant harmonically related Wave component in said wave, said segregating means including a plurality of impedance elements having reactances which are variable in accordance with variations in the magnitude of said derived frequency indication, means for recurringly sampling each segregated Wave component during a predetermined time interval, means for controlling the interval between samplings of adjacent components in accordance with the number of wave components to be sampled during each of said intervals, and means for combining said samples into a single indication the magnitude of which is indicative of the energy content in a respective one of said harmonically related Wave components at a designated instant.

13. In combination, a source of variable frequency complex wave energy comprising fundamental and harmonically related wave components, means for controlling the fundamental frequency of said complex wave to produce a Wave which varies in accordance with the frequency of the fundamental component of an original signal wave, means for segregating each harmonic. wave component of said produced wave, said means comprising a plurality of Variable frequency-selective networks each comprising an impedance element having reactance which is variable in accordance with changes in the frequency of the fundamental component of the original signal wave, means for modulating the relative amplitude of each segreated component in accordance with the relative amplitude relations existing between the harmonic components of the original signal wave, and means for combining said modulated harmonic components into a similitude of said original signal wave.

14. In a combination for analyzing and synthesizing a complex electric Wave which comprises a plurality of Wave components that are in harmonic relationship to a fundamental component, which comprises means for subdividing said Wave into two portions, means for deriving from a iirst portion an indication of the frequency of the waves fundamental component, variable frequency segregating circuits for dividing the second portion of said wave into its individual harmonically related components, said circuits including a frequency selective net- Work respective each harmonic component that may exist in the original signal wave, each of said networks includingr a variable impedance element having reactance which is responsive to a characteristic of the derived fundamental frequency indication, means for determining during recurring sampling periods the energy level of each of the segregated wave components, means for limiting said energy determinations during each sampling period to the harmonic components that exist in said signal wave during said period, means for combining said intermittent samples into a continuous spectrum-amplitude representation of said original signal wave and in which the amplitude at any instant is indicative of a respective energy level of a harmonic component at a corresponding instant, means for transmitting said spectrum-amplitude representation to the synthesizing location, a source of synthesizing signal energy having fundamental and harmonically related components, and in which the frequency of the fundamental component is maintained in substantial agreement with the frequency of the fundamental component in said original signal wave, means responsive to said derived fundamental frequency indication for individually segregating each harmonic component of said synthesizing energy, said means including a frequency selecting network respective each harmonic component that may exist in the original signal wave, each of said networks including an impedance element having reactance which is controlled by the characteristic of the derived fundamental frequency indication, means for modulating the amplitude of each of said segregated synthesizing harmonic components in accordance with respective instantaneous values of said spectrumamplitude representation, and means for combining said modulated components into a similitude of said original wave.

l5. In a combination for analyzing and synthesizing a complex electric wave which comprises a plurality of wave components in integral harmonic relation to a fundamental wave component, means for subdividing said wave into two portions, means for deriving from a iirst portion an indication of the frequency of the waves fundamental component, variable frequency segregating circuits for dividing the second portion of said wave into its individual harmonically related components, said circuits including a frequency selective network respective each harmonic component that may exist in the original signal Wave, each of said networks including a variable impedance element having reactance which is controlled by the magnitude of the derived fundamental frequency indication, means for determining during recurring sampling periods the energy level of each segregated wave component during that period, means for limiting the number of samples taken during each sampling period in inverse relation to changes in said derived frequency indication, means for combining said intermittent samples into a continuous spectrum-amplitude representation the amplitude of which at any instant is indicative of the energy level of a respective harmonic component at a corresponding instant, means for transmitting said spectrum-amplitude representation to the synthesizing location, a source of synthesizing energy having a fundamental and its harmonically related wave components, and in which the frequency of the fundamental component is maintained in substantial agreement with the frequency of the fundamental component in said original wave, means responsive to said derived fundamental frequency indication for segregating the individual harmonic components of said synthesizing energy, said means including a frequency selective network respective each harmonic component that may exist in said original signal wave, each of said networks including an impedance element having reactance which is controlled by the magnitude of the derived fundamental frequency indication, modulatory means responsive to the respective amplitude values of said spectrumamplitude representation for controlling the relative amplitudes of said synthesizing harmonic component, means for combining said modulated components, and means responsive to said derived fundamental frequency indication for controlling the number of modulated components that are so combined.

16. In a signal wave analyzing and synthesizing system, a means for sampling the amplitudes of the individual harmonic components of the signal wave which means comprises a normally unoperated sampling means, a pulse-producing means, means for deriving an indication ofthe fundamental frequency of the signal wave, oscillating means responsive to said frequency indication for actuating said pulse-producing means at a rate controlled by said frequency indication, and interconnecting circuit means for supplying said pulses to said sampling means for rendering said sampling means operative during said pulse periods. 17. In a signal Wave analyzing system, a means for individually sampling the amplitude of each harmonically related wave component that may exist in a signal wave during each interval of a series of consecutive equal intervals which comprises, a pulse actuated sampling means, a sampler-actuating pulse-producing means, means for deriving an indication of the fundamental pitch of the signal wave, and oscillatory means responsive to said pitch indication deriving means for actuating said pulse-producing means and said sampling means at a rate inversely proportional to the fundamental pitch of the signal wave.

18. In the method of analyzing and synthesizing signal waves the character of which varies from one syllabic interval to another, the steps of successively sampling during each syllabic interval and in the order of their frequency positicns the amplitude of each harmonic wave component existing in said signal wave during said interval, and the step of controlling the time interval between samples of adjacent wave components in accordance with the number of harmonic wave components existing in said signal wave during said syllabic interval.

19. In a signal wave analyzing system, a means for sampling the amplitude of each harmonically related wave component that exists in a signal wave during each interval of a succession of equal intervals which comprises, a pulse-operated sampling device respective each harmonically-related wave component that may exist in a given signal wave, means for deriving a voltage indication of the frequency of the fundamental cornponent of the signal wave, oscillatory pulse-producing means responsive to said frequency indicating means for producing sampler-actuating pulses the repetition rate of which is inversely proportional to said derived frequency indication, a periodic pulse producing means having a repetition rate equivalent to said equal intervals, and distributing means responsive to said sampler-actuating pulse-producing means and to said periodic pulse-producing means for supplying said sampler-actuating pulses during each equal interval to respective ones of said sampling devices in the order of the relative frequency relations of the harmonic components respective said sampling devices.

20. The method of operating on a variable frequency signal Wave the amplitudes of the Wave components of which are variable at a predetermined maximum rate, which method comprises segregating the components of said Wave in accordance with the frequency of said Wave, sampling the energy in each segregated portion at a fixed rate proportional to the maximum rate of said amplitude changes, varying the interval between samplings of adjaoently frequency-1ocated segregated portions in accordance With changes in the frequency of said wave, and combining said energy samplings in the order of their extractions into a single variable function that is representative of the relative amplitudes of the components of said signal Wave.

21. The method of operating on a variable frequency signal Wave, the amplitudes of the harmonically-related wave components of which are variable at a predetermined rate, which method comprises segregating the individual harmonically-related components of said wave, extracting a specimen of the energy content of each of said segregated components at a fixed rate which is proportional to the maximum rate of said amplitude changes, varying the interval between energy-specimen extractions from adjacent components in accordance with changes in the frequency of said wave, and combining said extracted energy specimens in the order of their extraction to form a single variable voltage representation of the relative amplitudes of said segregated components.

ROBERT C. MATHES.

References Cited in the le of this patent UNITED STATES PATENTS Number Name Date 2,093,956 Dudley Nov. 16, 1937 2,183,248 Riesz Dec. 12, 1939 2,243,089 Dudley May 27, 1941 2,269,295 Vaderson Jan` 6, 1942 2,286,072 Dudley June 9, 1942 2,339,465 Dudley Jan. 18, 1944 FOREIGN PATENTS Number Country Date 543,238 Great Britain Feb. 16, 1942 

